One of the most common reactions to Verizon's announcement that it will throttle the heaviest users of its "unlimited" 4G plans went something like this: "That's the last straw—I'm switching to T-Mobile!"

Unfortunately, switching to T-Mobile, AT&T, or Sprint won't protect you from getting throttled, even if the carrier is claiming to sell you "unlimited" data.

Let's take a look at the relevant passages in each carrier's terms and conditions. We'll start with the Verizon Wireless announcement last week:

Starting in October 2014, Verizon Wireless will extend its network optimization policy to the data users who: fall within the top 5 percent of data users on our network [using 4.7GB or more per month], have fulfilled their minimum contractual commitment, and are on unlimited plans using a 4G LTE device. They may experience slower data speeds when using certain high bandwidth applications, such as streaming high-definition video or during real-time, online gaming, and only when connecting to a cell site when it is experiencing heavy demand.
Ugh, that's awful! I'm switching to T-Mobile!

Well, before you switch, take a look at T-Mobile's policy:

Customers who use more data than 95% of customers on the same rate plan typically use in a month may, during times and places of congestion, have their data usage prioritized below other customers.
It's well known that T-Mobile throttles users who go over their 4G data limits. If you buy a plan with 1GB, 3GB, or 5GB of 4G LTE data, T-Mobile states that you'll be throttled to 2G speeds (up to 128Kbps) if you go over that allotment. "If total usage exceeds 5GB or the amount specified in your data plan during a billing cycle, we may reduce your data speed for the remainder of that billing cycle," T-Mobile says. Customers can purchase more high-speed data or use the slower data with no limits or overage charges.

But T-Mobile also sells an "Unlimited 4G LTE Data" plan for $80 a month. While this plan isn't automatically throttled after a user hits a pre-determined limit, the T-Mobile policy indicates that it may still be throttled "during times and places of congestion" if you're in the top five percent of users.

T-Mobile customers expecting to get unlimited high-speed data out of that plan have been in for a surprise. "Why is my unlimited 4G throttled?" one T-Mobile user asked in a customer support forum last year after seeing speeds drop while uploading large files.

An AnandTech forum user ran into a similar problem last year, detailed in a thread titled "PSA: T-Mobile's new unlimited, unthrottled 4G plans have a throttle point at 5-7GB." Other T-Mobile users said they weren't being throttled even after going well over those amounts, but that isn't a surprise since T-Mobile says throttling is limited to "times and places of congestion."

We asked T-Mobile this morning for more detail on how its throttling policy applies to unlimited 4G plans, but we haven't heard back yet.

UPDATE: A T-Mobile spokesperson told Ars, "we do not throttle our Unlimited customers. We do manage infrequent network congestion issues through prioritization of customer traffic." Like Verizon, T-Mobile seems to be claiming that slowing traffic down isn't "throttling" if it only happens occasionally. Ultimately, T-Mobile is doing the same occasional throttling of heavy users that other carriers do.

AT&T and Sprint also throttle people who use 5GB or more

AT&T and Sprint are no better, which probably will not surprise you.

AT&T offers advice on how to "avoid reduced speeds with unlimited data plans":

Customers with a 3G or 4G smartphone who have an unlimited data plan may see speeds reduced as a result of AT&T network management practices if they exceed 3GB of data in a billing cycle. For customers with a 4G LTE smartphone who have an unlimited data plan, speeds will be reduced if usage exceeds 5GB in a billing cycle.
More than 97 percent of smartphone customers are not affected, AT&T says. The throttling is limited to times and places where the network is congested, AT&T says.

Similarly, Sprint says the following:

Sprint currently employs prioritization to improve data experience for the vast majority of users on Sprint’s CDMA and LTE networks. The heaviest data users consume a disproportionate share of network resources and cause a negative user experience for the rest. To more fairly allocate network resources in times of congestion, customers falling within the top 5% of data users may be prioritized below other customers attempting to access network resources, resulting in a reduction of throughput or speed as compared to performance on non-congested sites. Though the exact amount of data used before a customer falls within the top 5% of data users will vary from month to month, currently customers who typically use 5 GB or more in a given month are likely to fall within the top 5% of data users.
Throttling comes standard

The carriers' policies make it obvious that throttling heavy users, particularly in congested areas, is standard. "Reducing data speed (data throughput) and network management are common practices in the wireless industry," AT&T notes.

This isn't automatically a bad thing if it's necessitated by limited bandwidth and done only when cell sites are so congested that heavy users would prevent lighter users from connecting at all. But there can be different motives. Verizon's announcement was a little curious in that it affects only users who "have fulfilled their minimum contractual commitment."

Verizon stopped offering unlimited plans in 2011, but there are customers with grandfathered unlimited data who have paid off their contracts but continue to purchase Verizon service on a month-to-month basis. Verizon is throttling those users when they're on congested cell sites if they use 4.7GB or more per month, even though those same users could get 6GB or more per month if they signed a new (pricier) contract. Signing up for a limited plan would also open those users up to overage fees.

Verizon has been more successful than its rivals at pushing customers onto limited data plans, and throttling out-of-contract users is just one more tactic to convert the remaining holdouts.